1. Field of the Invention
This invention pertains to the determination of an orientation associated with a mobile system, for instance a helmet visor and corresponding apparatus, for determining an orientation associated with a mobile system with greater accuracy.
2. Description of the Related Art
The question involves accurately pinpointing the orientation of a directed axis without having physical contact with it, such as helmet visor systems used in fighter aircraft or helicopters. The operation of such a system is briefly recalled herein: a pilot sees, with an ancillary sighting device, and through a half-reflecting pane that is attached to the helmet and set on a vision axis, the image of a reticula is projected at infinity and superimposed on the outside scene. When the pilot wants to designate a target, the pilot moves the reticula to coincide with the target and he signals that the coincidence is executed, for instance with a push-button control that was conceived for this purpose.
Provided that the precise orientation of the helmet is pinpointed when the reticula coincides with the target, it is possible to determine, in relation to the aircraft, the sight orientation and to designate the target for a weapons system, or to point an optical system or other system in that direction.
French patent no. 79 14441 discloses an orientation measure apparatus, for a helmet visor system. To bring the line of sight determined by the position of the helmet of the pilot into a coordinate frame associated with the aircraft, a position pick-up comprised of three electro-magnetic orthogonal coils is placed on the helmet. At a fixed point in the cabin, there are three other electro-magnetic coils, which form a static transmitter or "radiator" according to the terminology in use in that field. Then is successively made to electric current, cross, through each coil of the transmitter (which forms the static trihedron). These currents make three magnetic fields appear which are picked up by the coils of the pick-up (which forms the mobile trihedron associated with the helmet).
For each magnetic field that is emitted, the three corresponding electro-magnetic orthogonal components are picked up inside the coils of the pick-up. The nine resulting components make it possible to find the transformation (rotation) that enables passage from the mobile trihedron to the static trihedron. Indeed, those nine components rely:
on the position of the pick-up in space; and PA1 the orientation or pure notation of the pick-up.
The above method is flawed. The transformation that results from the above method can be quite different from the actual transformation of the coordinates which allow effective passage from the mobile reference trihedron to the static reference trihedron in relation to the aircraft, especially because magnetic fields are used. Indeed, those fields can be disrupted because of metallic elements in the cabin environment, creating reflections of magnetic waves inside the cabin. Furthermore, flaws inherent to the execution of emitting or receiving magnetic, radiation also create disturbances.
Determining an orientation by using the laws of electromagnetism in free space is, therefore, not possible when the space under consideration is the piloting cabin of the aircraft. Some familiar devices build into memory storage, prior to each measurement, preliminary measurements representing electromagnetic disturbances in the cabin. Comparison of the preliminary measurements carried out at a given time with the measurements stored in memory determines the rotation that allows for passage from the static trihedron to the mobile trihedron. However such a method requires storing in memory a very substantial volume of measurements which must be completely recalled as soon as parameters change, such as the exact position of the mobile emitter in relation to the line of sight.
The invention provides a method and corresponding apparatus for determining an orientation inside a mobile system that makes it possible to use in operation the same measuring method, previously discussed, without the above-mentioned flaws.
To this end, in a preliminary phase, modeling takes into account the disturbances in particular of magnetic origin, inside the cabin wherein the sensing system is placed. The modelling is set up based on specific known directions of the mobile pick-up or on specific known directions of the line of sight and for each direction based on a set of m positions, the origin of the mark comprised of the mobile pick-up. This preliminary phase, based on real measurements, makes it possible to broach the complex relations between position and orientation resulting from the reflecting of magnetic waves and the physical limitations of the emitter and the pick-up. This preliminary phase leads to the preliminary determination of a global modeling function by factoring in disruptive effects. This preliminary phase, ultimately completed through initialing during flight, makes it possible in operation to directly determine the direction associated with any position of the mobile system, based on real experimental measurements.
According to the invention, a method for determining an orientation associated with a mobile system inside a given space, by using a magnetic system for position and orientation sensing, is provided. An emitter made of three orthogonal coils that have a static position inside the given space and a pick-up made of three orthogonal coils affixed to the mobile system are included, r being the distance between the origin of the emitter mark and the origin of the pick-up mark. A sensing cycle consisting of the successive emitting in the three coils of the emitter and of measuring the field components generated in the coils of the receiver, is characterized by a preliminary modeling phase of the given space which consists of conducting measurements of the magnetic fields in the pick-up for a set of known positions of the mobile system in relation to a mark known as the cartographic mark. Modelling parameters are deduced from the reference measurements at each point being computed to deduce therefrom the relative rotation of the pick-up in the cartographic mark of the reference measurements. Thus, the instant orientation of the mobile system is directly determined based on modeling parameters comprising a compact modeling function position of the disruptive effects, which area function of the position.
The invention also provides an apparatus aimed at implementing this method.